System for regulating the active power transferred into and out of direct voltage network by multiple power stations

ABSTRACT

A system for transmitting power over a direct voltage network. A first power station connects one alternating voltage network to the direct voltage network. The first power station includes a converter for bidirectionally transferring power between the direct voltage network and the alternating voltage network. The converter maintains constant value of voltage on the network, or operates in a second alternative mode of controlling constant current between the alternating voltage network and the direct voltage network. When the voltage on the direct voltage network drops below a reference value, a second power station connected to the direct voltage network and to a remaining alternating voltage network takes over the voltage regulation function. The second power station prior to taking over the voltage regulating friction was in a current regulating function. In this way, when the first station reaches its limits of voltage regulation, the second station is enabled to take over the voltage regulating function switching out of a current regulating function.

BACKGROUND OF THE INVENTION

The present invention relates to a system for transmitting electricpower comprising a direct voltage network, and at least two alternatingvoltage networks which are connected to the direct voltage networkthrough a respective power station. The power stations transfers powerbetween the direct voltage network and the respective alternatingvoltage network, and has at least one VSC converter to convert directvoltage into alternating voltage, and conversely convert alternatingvoltage to a direct voltage. An apparatus in a first station controlsthe converter of the station to regulate the direct voltage of thedirect voltage network and maintains the direct voltage at apredetermined nominal value. An apparatus of the other station controlsthe converter of that station for regulating the current flowing betweenthe respective connected alternating voltage network and its connectedstation.

Such a system is known through the thesis "PWM and Control of Two andThree Level High Power Voltage Source Converters" by Anders Lindberg,Kungliga Tekniska Hogskolan, Stockholm, 1995, which describes a systemfor transmitting electric power through a direct voltage network forHigh Voltage Direct Current (HVDC). It is emphasized that the inventionis not restricted to this application, but for purposes of illustrationthe invention will be described with respect to this known system.

Prior to the system described in the thesis, systems for transmittingelectric power through a direct voltage network for High Voltage DirectCurrent have been based upon the use of line-commutated CSC (CurrentSource Converter) converters in the power stations. The development ofIGBTs (Insulated Gate Bipolar Transistor=bipolar transistor having aninsulated gate) for high voltage applications which may be easily turnedon and turned off simultaneously, has made it possible to create valvesfor VSC (Voltage Source Converter) converters. VSC converters permitforced commutation as an alternative. This type of power transmissionbetween a direct voltage network for High Voltage Direct Current andalternating voltage networks connected thereto offers several importantadvantages over the use of the prior art line-commutated CSCs used inHVDC. Th VSC (Voltage Source Converter) permits the consumption ofactive and reactive power to be controlled independently of each other,and there is no risk of commutation failures in the converter and nocorresponding risk of transmitting commutation failures betweendifferent HVDC links, which may take place in line-commutated CSC.Furthermore, there is the possibility of feeding power to a weakalternating voltage network or a network which doesn't generate its ownpower (a "dead" alternating voltage network).

In a system of the type discussed above for HVDC with VSC converters,the direct voltage of the direct voltage network is determined in afirst power station, and it is desirable to control the direct voltagelevel without any rapid telecommunication between a station havingvoltage-regulating apparatus and the other stations along the directvoltage network. Since each power unbalance results in a change of thedirect voltage, which is then corrected by the voltage-regulatingstation, such a communication possibility will be superfluous. Shouldthe power fed into the network drop as a consequence of limitations inan alternating voltage network for feeding power to a direct voltagenetwork, or in the voltage-regulating station, or should the power fedout of the network exceed the available power fed in, the direct voltageof the direct voltage network will drop. The direct voltage will drop somuch that the power fed out is reduced to such a level that powerbalance is reestablished. This means that the voltage-regulating stationhas arrived at its regulation limit and cannot maintain the directvoltage at said determined nominal value. This may result in collapse ofthe direct voltage and temporary disabling of parts of the HVDC system.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a system which solvesthe problems which arise from differences between the active power fedinto the direct voltage network and the active power fed out therefrom.

The object according to the invention is realized by the fact that insuch a system the regulating apparatus of the first power station aswell as of the other power stations, are adapted to enable regulation ofeither the voltage or the current of the direct voltage network. Thesystem comprises means connected to each station for measuring thedirect voltage on the direct voltage network, and each power station isprovided with means to compare the direct voltage measured at thestation, with a predetermined value. When the measured voltage is lessthan the predetermined value, it sends signals to the respectiveapparatus for changing regulation mode. When the apparatus is incapableof regulating the direct voltage close to the predetermined value, acurrent-regulating station takes over voltage regulation from thepreviously voltage-regulating station which in turn becomes currentregulating.

Enabling regulation of either voltage or current of the direct voltagenetwork should be interpreted as having regulating apparatus in thesepower stations which may be in one or the other of these regulationmodes at a time. In order to regain power balance upon a powerunbalance, any of the stations other than the first voltage-regulatingstation, has to increase the active power flow towards the directvoltage network, or reduce the tapping of power from the direct voltagenetwork. Since a second station may according to the invention take overthe voltage regulation from the first one, and thus enter into so-calledback-up voltage-regulating mode, when the direct voltage of the directvoltage network falls below a predetermined value, a further collapse ofthe direct voltage may be avoided and power balance may be regained.

According to a preferred embodiment of the invention, the respectivestation comprises members which measure the time during which the directvoltage of the voltage-regulating power station has a value below thepredetermined value. The stations are adapted to return to theregulating modes they were in before this reduction in voltage as soonas the reduction in voltage has ceased when the reduction was only atransient reduction in the direct voltage. Thus, the power station whichis initially voltage regulating gets the voltage-regulating functionback when only transient voltage limitations occur. This is anadvantage, since there are usually good reasons for selecting a certainstation of the system as a voltage-regulating station, for example whenthis power station is connected to the most powerful alternating voltagenetwork.

According to another preferred embodiment of the invention therespective station comprises means to measure the time that the directvoltage at the regulating power station has a value below saidpredetermined value, and the power stations make a change of theregulating mode of the power stations permanent when this time exceeds acertain comparison value. This means that upon a remaining non-transientvoltage drop, the voltage-regulating station is permanently changed toanother power station, since the regular voltage-regulating station cannot overcome its power regulating limitations.

According to a preferred embodiment of the invention, the new powerstation which is voltage regulating, following a permanentregulating-mode change, assumes the reference voltage of the directvoltage network, which was used by the previously voltage regulatingpower station. In this way it is ensured that the new voltage-regulatingstation receives the same target regulation voltage of the directvoltage network as the previously voltage-regulating station had. It ispossible that a predetermined direct voltage value of said new powerregulating station was lower than said reference value, since it ispossible that a lower voltage level is normally allowed at the stationsbeing not voltage regulating than at the voltage-regulating station.

According to another preferred embodiment of the invention the means tocompare of the first power station chooses the predetermined voltagelevel as a value lower than 95% of a nominal value of the directvoltage. This is advantageous, since otherwise a change ofvoltage-regulating station could occur upon minor variations of thedirect voltage which is not required. More particularly, it may beadvantageous that the level be within the interval 85-95% of the nominalvalue. It is important to intervene and change the voltage-regulatingstation when the direct voltage has fallen so much below the nominalvalue so as to prevent costly disturbances of the plant.

According to another preferred embodiment of the invention theapparatuses are adapted to have a time of about 0.2 seconds as thecomparison value. It has been found that if the voltage remains belowthe nominal value for more than 0.2 seconds, it is time to permanentlychange to another voltage regulating station.

According to another preferred embodiment of the invention the systemcomprises more than two power stations for connecting alternatingvoltage networks to the direct voltage network, and more than two of thepower stations have a regulating apparatus thereof adapted to enableregulation of either voltage or current. It is thus possible to let asuitable station take over the voltage regulation at each separateoccurrence of any power unbalance. It is then advantageous for all thepower stations to have their regulating apparatus adapted to enableregulation of either voltage or current According to a furtherdevelopment of this embodiment with more than two stations, thedifferent power stations are adapted to change to voltage-regulatingmode when different predetermined values of the direct voltage at therespective station are detected. According to another furtherdevelopment of the embodiment having more than two stations, theregulating apparatuses of the different stations permanently change tovoltage-regulating mode when the measured voltage falls below thereference values for different durations in time. These embodiments maybe customized both with respect to level of voltages for a reference andthe time interval of a low voltage condition for changing regulationmodes for the different stations i.e. the voltage regulation of thedifferent stations is activated at different reference levels and becomepermanent when the voltage remains below the respective reference levelat different times.

According to still a further preferred embodiment of the invention thesystem is adapted for transmitting electric power through a directvoltage network for High Voltage Direct Current (HVDC). The advantagesof the system according to the invention are particularly apparent inthis preferred application.

Further advantages as well as advantageous features of the inventionwill appear from the following description and the other dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the principles of theinvention; and

FIG. 2 consists of graphs illustrating the progress of some electricquantities with the time of occurrence of a power unbalance andcorresponding change of the voltage-regulating station.

DETAILED DESCRIPTION OF THE INVENTION

The system for transmitting electric power according to a preferredembodiment of the invention is schematically illustrated in FIG. 1, inwhich only the components directly related to the function according tothe invention have been shown to facilitate an understanding of theinvention. The system comprises a direct voltage network 1 for HighVoltage Direct Current (HVDC=High Voltage Direct Current) and in thepresent case four alternating voltage networks 6-9 connected to thedirect voltage network through a respective power station 2-5. Thealternating voltage networks are indicated through an alternatingvoltage symbol and an inductance. The stations are adapted to performtransmission of electric power between the direct voltage network 1 andthe respective alternating voltage network in which power may be fed infrom an alternating voltage network to the direct voltage network or fedout from the direct voltage network to an alternating voltage network.The alternating voltage networks may accordingly have generators ofelectric power or only be connected to power consumers thereof. Thepower stations comprise at least one VSC converter 10-13 adapted toconvert direct voltage into alternating voltage and converselyalternating voltage into direct voltage. However, it is possible thateach station comprises several such converters, but these arc in thepresent case summarized by a single box for each station. It is alsopossible that the alternating voltage networks have more than one phase,usually three phases, but the phases of the alternating voltage networksare in the figures summarized through a single line. The respective VSCconverter includes conventional valves which are branches of breakersconnected in series which can be electronically turned on and off, suchas IGBTs, connected to anti-parallel diodes. A large number of IGBTs maybe connected in series to form one single valve so as to be turned onand turned off simultaneously to function as one single breaker, and thevoltage across the valve is distributed among the different IGBTsconnected in series. The control of the breakers takes place in aconventional way by pulse width modulation (PWM).

The power stations further comprise an apparatus schematically indicatedas 14-17 for controlling the respective converter 10-13. Each stationhas means 18-21 connected for measuring the direct voltage, and thesemeans send information about the level of the direct voltage to a member22-25 to compare the direct voltage thus measured at the station with apredetermined value. The respective apparatus 14-17 is connected to therespective converter 10-13 of the station through a switching member26-29 which may be influenced between two positions by the comparingmember 22-25 so as to transfer the apparatus between avoltage-regulating and a current-regulating mode.

Only station 2 is in a voltage-regulating mode, where switching member26 is in a different position than the corresponding switching members27-29 of the other stations which are in the current-regulating mode.The regulation takes place in a conventional way by the way in which thecontrol pulses to the different valves of the converters are designed.The voltage-regulating station 2 attempts to keep the voltage of thedirect voltage network at a predetermined nominal value, but it ispossible that a power unbalance occurs in the system where the activepower fed into the direct voltage network differs from active power fedout of the network. Should the active power fed in fall as a consequenceof limitations of the feeding alternating voltage network, or from alimitation of the voltage-regulating station 2, or should power fed outexceed the available over fed in, the direct voltage of the network 1will fall. The direct voltage will then fall until the power fed out hasbeen reduced to such a level that power balance is there again. Thus,the voltage-regulating station 2 has arrived to the limitation thereofand cannot do anything more to maintain the nominal direct voltage onthe direct voltage network. Accordingly, the other stations 3-5 mustincrease the active power flow towards the direct voltage network, orreduce the tapping of power therefrom so as to attain power balance. Inorder to obtain this there is in each station a back-upvoltage-regulating mode, which is effective when the direct voltagefalls below a predetermined level, preferably about 90% of the desiredlevel, and then takes over the direct voltage regulation. This isachieved by the respective comparing members 22-25 which compare thevoltage measured by the associated voltage measuring means 18-20 with apredetermined value. When the measured value compares lower than thepredetermined reference value, the comparing means sends signals to therespective apparatus, more exactly the switching members 26-29, so as tochange the regulation mode. Upon the voltage-regulating apparatusbecoming incapable of maintaining the direct voltage close to thenominal predetermined reference value, one of the previouslycurrent-regulating stations takes over the voltage regulation from thepreviously voltage-regulating station, which then becomes currentregulating. Accordingly, if the station 3 takes over the voltageregulation, the switching member 27 will turn clockwise, while theswitching member 26 turns counterclockwise. In the respective comparingmember 22-25, members for measuring the time during which the directvoltage of the station has a value below said predetermined referencevalue are also included, and in the event of a transient, i.e., arapidly passing reduction of the direct voltage at the station beingvoltage regulated, the stations return to the regulation modes they hadas soon as the voltage reduction has disappeared. This is desired sincethere is often a desire to have a certain station as thevoltage-regulating station. However, said time measuring members ensurethat a permanent change of regulation mode between the stations takesplace should the power unbalance problem remain, i.e., should it stillbe there after a certain time, for example about 0.2 seconds. If such apermanent change takes place the voltage reference of the newvoltage-regulating station is increased to said nominal value of theprevious voltage regulation station since the voltage reference of thisstation, with which it is compared in said comparing member, normally isset to a value being lower than said nominal value. Because of thepossibility of transferring the voltage regulation to a station whichhas not reached its regulation limit, a power unbalance may beefficiently corrected.

There is therefore no requirement of any rapid telecommunication betweenthe different power stations thanks to the detecting of the powerunbalance by measuring the level of the direct voltage on the directvoltage network at each of the power stations.

It is preferred that only one of the power stations is voltageregulating at a time, while the other stations are current regulating,but in a system having a great number of stations more than one stationcould be voltage regulating at a time. It is preferred that all thestations have the capability to function both in a current-regulatingmode and a voltaic-regulating mode, so that all the stations mayfunction as back-up stations, but it is possible that any or some of thepower stations may function only in the current-regulating mode.

FIG. 2 illustrates with line 30 how the active power is fed out from thevoltage-regulating station 2 over time. Lines 31 and 32 show,respectively, the progress of the direct voltage and the active powerfed in by the station changing into the voltage-regulating mode. At thetime represented by point 33, the voltage-regulating station reaches itslimitation and the active power is reduced there, and the direct voltage31 falls. When the direct voltage has fallen below a predetermined level34, which is preferably 90% of the previous direct voltage , the otherstation changes to the voltage-regulating mode, which is indicated bythe point 35. Accordingly, the active power being removed from thedirect voltage network is reduced at this station to obtain powerbalance.

The direct voltage has at the point 36 been returned to the nominalvalue by the new voltage-regulating station.

The invention is of course not in any way restricted to the preferredembodiment described above, but many possibilities of modificationswould be apparent to the man skilled in the art without departing fromthe basic idea of the invention.

Many such alternatives have been mentioned, but it is once againrepeated that different direct voltage values may be accepted atdifferent stations, and that the system has components not shown in thedrawing, such as for example harmonic filters for removing harmoniccurrents arising in the pulse width modulation.

Although symbols have been shown in FIG. 1 for certain members, means orthe like, it is not necessary that these exist as separate components,but their respective functions may be realized with componentsperforming multiple functions. Some values may for example not bemeasured directly but calculated from the values of any other quantitymeasured.

The foregoing description of the invention illustrates and describes thepresent invention. Additionally, the disclosure shows and describes onlythe preferred embodiments of the invention, but as aforementioned, it isto be understood that the invention is capable of use in various othercombinations, modifications, and environments and is capable of changesor modifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings, and/or the skill orknowledge of the relevant art. The embodiments described hereinabove arefurther intended to explain best modes known of practicing the inventionand to enable others skilled in the art to utilize the invention insuch, or other, embodiments and with the various modifications requiredby the particular applications or uses of the invention. Accordingly,the description is not intended to limit the invention to the formdisclosed herein. Also, it is intended that the appended claims beconstrued to include alternative embodiments.

What is claimed is:
 1. A system for transmitting power comprising:adirect voltage network; at least two alternating voltage networks; afirst power station connecting one of said alternating voltage networksto said direct voltage network including:(i) a VSC converter forbi-directionally transferring power between said direct voltage networkand one of said alternating voltage networks, (ii) control means forselectively controlling said VSC converter so that either a constantvalue of direct voltage is maintained on said direct voltage network, ora constant current is maintained between said alternating voltagenetwork and said direct voltage network, (iii) means for comparing thevoltage on said direct voltage network with a nominal value, and forcontrolling said VSC converter to maintain said voltage on said directvoltage network at said nominal value, and (iv) means for switching saidmeans for controlling into a current controlling mode if said VSCconverter fails to restore said direct voltage network to said nominalvalue of direct voltage, a second power station connecting said directvoltage network to a remaining of said alternating voltage networksincluding,(i) a VSC converter for bi-directionally transferring powerbetween said direct voltage network and said remaining alternatingvoltage network, (ii) means for comparing said direct voltage on saiddirect voltage network to a nominal voltage value, (iii) control meansfor selectively controlling said second power station VSC converter sothat either a constant value of direct voltage is maintained on saiddirect voltage network, or a constant current is maintained between saiddirect voltage network and said remaining alternating voltage network;and (iv) switching means connected to said means for selectivelycontrolling said second power station VSC converter for switching saidVSC converter from a current controlling mode to a voltage controllingmode when said means for comparing indicates that said first powerstation has failed to maintain said direct voltage at said nominalvoltage value.
 2. The system according to claim 1 wherein said means forcomparing in said first and second power stations initiates switching ofsaid VSC converters from one mode to another when said measured directvoltage is less than said nominal voltage value for a predetermined timeinterval.
 3. The system according to claim 1 further comprising:means atsaid first station for changing said mode back to a voltage controllingmode, and means at said second station for changing said mode ofoperation back to a current controlling mode if said direct voltage isreestablished within a predetermined time.
 4. The system according toclaim 1 wherein said second power station after switching to a voltagecontrolling mode regulates said direct voltage to equal the nominalvoltage previously used by said first station to regulate said directvoltage.
 5. The system according to claim 1, wherein the first stationnominal voltage value is lower than 95% of a standard value of directvoltage.
 6. The system according to claim 5, wherein said first stationnominal voltage value is within 85-95% of the standard value of thedirect voltage.
 7. The system according to claim 2, wherein saidpredetermined time is longer than 0.15 seconds.
 8. The system accordingto claim 2, wherein said predetermined time is substantially 0.2seconds.
 9. The system according to claim 1 wherein said power stationsare configured so that only one is in the voltage regulating mode at onetime.
 10. The system according to claim 1 further comprising:a thirdpower station connected to an additional alternating voltage network andto said direct voltage network comprising:converter means forbidirectionally transferring power between said direct voltage networkand said additional alternating voltage network, means for comparingvoltage on said direct voltage network with a reference voltage value,means for selectively controlling said converter means so that either aconstant value of said voltage on said direct voltage network ismaintained or a constant current is maintained between said directvoltage network and said additional alternating voltage network, andcontrol means connected to said means for selectively controlling saidconverter for switching said converter from a current controlling modeto a voltage controlling mode when said direct voltage is less than saidreference value.
 11. The system according to claim 10 wherein saidreference value of said third power station is less than the nominalvoltage value of said second power station.
 12. The system according toclaim 11, wherein said second power station control means connected tosaid means for selectively controlling said converter switches saidconverter to a current controlling mode when said third power stationswitches to a voltage controlling mode.
 13. The system according toclaim 11, wherein said third power station switches to a voltageregulating mode when said measured direct voltage is less than saidnominal voltage for a predetermined time.